MikeBegley:
One other suggestion. When playing campaigns (especially military) I try and guess where the enemy might appear and keep those entry points open and clear of any buildings.
Then when the enemy attacks he doesn't have the advantage of being right upon your city, and you have the time to send your troops to do battle.
Also, in the harder campaigns you can often fortify those open areas with walls and towers.
Hope this helps,
Vriesea